The concept of producing pulsed metal vapor laser transitions by placing a metal halide of the desired metal within an enclosure and vaporizing the metal halide to provide a metal halide vapor is described in detail in U.S. Pat. No. 3,934,211, issued Jan. 20, 1976, entitled "Metal Halide Vapor Laser", and assigned to the assignee of the present invention. This U.S. patent is incorporated herein by reference. There is further disclosed in U.S. Pat. No. 3,936,772 issued Feb. 3, 1976, entitled "High Flow Metal Halide Vapor Laser", a technique for obtaining high pulse rate metal vapor laser transitions. This latter patent is assigned to the assignee of the present invention and is also incorporated herein by reference.
In accordance with the above-identified U.S. Pat. No. 3,934,211, the metal halide vapor thus produced is thereafter disassociated to provide ground state metal atoms of sufficient number density to create a condition for resonance trapping and substantially simultaneous therewith, ground state metal atoms are excited to an upper laser level, while maintaining a sufficient number in the ground state to preserve the resonance radiation trapping condition, with electrons sufficiently energized to create a population inversion between the upper and lower laser level. The excited metal atom is permitted to emit laser radiation by stimulated emission to a lower laser level and the emission is radiated, preferably between a pair of externally mounted mirrors. The metal atom is permitted to relax from the lower laser level to the ground state and the aforementioned steps are repeated. U.S. Pat. No. 3,934,211 identifies as well known the metal vapor lasers of copper, manganese, and lead.
As disclosed in U.S. Pat. No. 3,934,211, it was determined that the metal component of a metal halide molecule can be made to lase at temperatures substantially below those required for pure metal vapors. It was further determined that thermal energy at or below those normally employed in pure metal vapor lasers do not provide adequate atomic densitites and, therefore, it is necessary to provide collisional excitation energy with energetic electrons to obtain dissociation of the molecular vapors.
While pure copper, lead, and manganese readily exhibited the operational characteristics of desired metal vapor laser transitions, thus rendering them prime candidates for modification to produce corresponding metal halide vapor lasers suitable for operation at lower temperatures, attempts to achieve pure bismuth metal vapor laser transitions proved unsuccessful. The lack of success of achieving successful operation of a pure bismuth metal laser is attributed primarily to the fact that the metal dimer, or diatomic molecule, Bi.sub.2 is formed by the pure bismuth metal laser and the metal dimer Bi.sub.2 absorbs substantial laser energy at the preferred underwater transmission wavelength of 4722 Angstrom thereby destroying the laser gain required to support the stimulated laser output.
The use of pure bismuth vapor to produce laser action at 4722 Angstroms has been unsuccessful to date because of a fundamental problem in bismuth volatilization, namely the formation of the diatomic molecule Bi.sub.2 in the vapor in addition to atomic Bi. For instance, at 1200.degree. K the ratio of molecular Bi.sub.2 to atomic Bi vapor pressure is about 1 to 3. Thus at the temperatures required to achieve even the minimum Bi densities for laser action, greater than 25% of the particles are in the molecular form. These molecules absorb electrical excitation energy at the 4722 Angstrom atomic radiation, thus rendering pure bismuth metal a poor candidate for lasing.
Inasmuch as the pure bismuth metal laser failed to achieve the operational status of the pure copper, manganese, and lead lasers, it has been generally concluded that the lower operating temperature benefits realized by the addition of metal halide molecules as taught by U.S. Pat. No. 3,934,211 was of no benefit to the pure bismuth laser.